Based on my research, and forum responses that seem well reasoned, I need an oil with an hot viscosity of 40 or 50, and a cold viscosity as low as possible (for ease of starting and initial start flow). Something like 15w40 in conventional, or 5w40 in synthetic. Pretty much confirms what most people recommend.

DixeyJulie, to have an engine seize through overheating once is bad, twice is terrible!

You say you have water flow through the exhaust, so the raw water side seems OK. However, what is the thermostat like? You could remove it and try running without it. However, if your engine has a sealed (freshwater) system which is being cooled via a heat exchanger, there could be a problem in that side of the system. Low coolant level, an air lock, circulating pump shot. An air lock? Sometimes happens after the coolant has been changed.

I doubt the wrong grade of oil could cause overheating like you describe. If there was a lack of oil pressure due to a blockage, failed lube pump, low oil level or foaming you should get an alarm or red light warning in time to shut down.

When you do shut down make sure you let it idle for a few minutes first. Although Yanmar recommend running the engine up to high RPM if it has not been working very hard, they still expect that after having done that, the operator will bring it back to idle for a few minutes to stabilise temperatures.

Thanks Smurphny,
Last week, I ran the motor in idle, in neutral for about 25 minutes, it had plenty of water flow in exhaust, Then, I thought it might be getting too hot,
So I reved it to 2500 for 15 seconds, then lowered RPM to idle in 5 seconds,
and shut it down. For some reason, the motor was very HOT, and it froze for about 2 hours. Since, I did not know it froze, I tried to start it after about 20 minutes after I shut it down, and It blew the starter aluminum cast shroud over the gear teeth into two pieces. Luckily, since this happened once before,
I had bought two, to have a spare, and replaced it. No aluminum cast pieces went into the flywheel cavities. But, does anyone know why it got so hot,
while running in idle?

Overheating to the point of seizing up is really bad. It's likely the cylinder walls have been scored already. You can't do this too many times until the engine is toast. If it will turn over again: 1. Change the oil and put in some Rotella straight 30 weight. Look at the old oil to see if it's milky which will mean water is getting into the oil somehow. This could cause the overheating. 2. Drain the anti freeze. (I would take the end cap off the heat exchanger to see if something is clogging it) If it's a standard raw water cooled engine ignore this. 3. Replace the thermostat or just take it out until you are sure you're getting coolant circulation. 4. Refill and start again but don't let it get that hot again.

Personally, I'd probably be pulling the head off after two seizings to check for cylinder damage before taking a chance running it again. This would also let you look at the water jackets to see if they are full of silt or something. The fact that water is running through the exhaust manifold doesn't necessarily mean enough water is running through the block.

(edit: I replied from the original post on your over heat on page 1, didn't see the previous replies until just now. I repeated a couple things, but also added a couple. No matter what you need to get to the bottom of this before using this engine again... if its still usable.)

Ouch!

Cooling circulatory issue. Raw water cooled? or Freshwater cooled?

If raw water cooled, I'm not sure, but I believe there is a thermostat that provides coolant flow to the cylinder water jacket... it bypasses what it doesn't need to the exhaust (so you'd still see flow even though its not being circulated through the engine). I'd bet that thermostat stuck closed, or failed to open enough for whatever reason. Line blockage post the thermostat is also a candidate as severe corrosion in the water jacket itself that can break free and plug the flow downstream so it prevents circulation.

If freshwater cooled... the raw water outflow from the heat exchanger does the exhuast cooling so it would still show in the exhuast. In this case, circulation was prevented in the freshwater side of the system (water pump, inoperative thermostat, pinched of or blocked line (blocked is unlikely though in the freshwater side of the system, but a pinched line due to fatigue is still possible). Its also possible in this case the the heat exchanger is tired... but unlikely.

Does your system include a temp gauge? Worth their weight in gold! Also some systems have a high temp shut down built it... if yours does...it must have failed too.

Obviously I am not looking at your engine... but from your description, its what it sounds like a thermostat failure or blockage to me or at least a good possibility of it.

Even though you replaced the starter... seizing likely scored the cylinder walls... while it may still run... it will start taking oil and have a reduction in power. Time for a rebuild or at least a refresh honing and new rings on the piston.

Thanks again, it sounds like I need to check the water thermostat.
The starter had broken once before (About 6 months ago) due to, trying to get the motor started with ether. I had been having some problems with starting and a mechanic got it started with ether once. Well, somehow, it was too much, and cracked the starting casing. on that starting effort. (Now, don't laugh too loudly at my ignorance). I am just learning diesels, for the first time.
So, that was the reason the starting cracked. The motor has run fine, (for some reason I cannot explain, and started on first turn of the key ever since; except for the recent episode of getting hot, at idle. (The motor did, once, have the warning heat alarm sound off a month ago), when my wife got confused on which way the thru haul raw water intake valved turned. She turned it off, and thought she had turned it on). So now, I have drawn a picture beside the thru haul valve for the raw water for ON and OFF positions. But, on this occasion, the warning heat sensor alarm did not sound off. I am wondering if a plastic bag might have been sucked to the the outside of the raw water intake. But, now, it could be the thermostat needs checked. I have since, ran the motor for over 20 hours, with over 15 different starts, up and down the ICW in Florida, travelled over 70 miles, and have not had the least bit of motor problem. But, I was glad to have ordered the extra starter (since I switched it out, in 20 minutes and was going again), without needing to call for a tow.
Is it much work to pull the head? I have never done it. And I guess the pattern of re-tightening the headbolts and the torque is in the manual? Right? And, what does scarring look like on the cylinders? Are there any pics on youtube that show a scarred cylinder. Wouldn't I have to pull off the block in order to replace the rings? Does a compression test simply tell me if the rings are shot? It still starts easily now on the first turn of the key.
Is that a good indicator of good compression? The rating of the 3GMD is 20 HP at 3400. The RPM goes to 3600, when running at full throttle. And I move at 7.3 knots. Haul speed is 7. Thanks so much!

Thanks again, it sounds like I need to check the water thermostat.
The starter had broken once before (About 6 months ago) due to, trying to get the motor started with ether. I had been having some problems with starting and a mechanic got it started with ether once. Well, somehow, it was too much, and cracked the starting casing. on that starting effort. (Now, don't laugh too loudly at my ignorance). I am just learning diesels, for the first time.
So, that was the reason the starting cracked. The motor has run fine, (for some reason I cannot explain, and started on first turn of the key ever since; except for the recent episode of getting hot, at idle. (The motor did, once, have the warning heat alarm sound off a month ago), when my wife got confused on which way the thru haul raw water intake valved turned. She turned it off, and thought she had turned it on). So now, I have drawn a picture beside the thru haul valve for the raw water for ON and OFF positions. But, on this occasion, the warning heat sensor alarm did not sound off. I am wondering if a plastic bag might have been sucked to the the outside of the raw water intake. But, now, it could be the thermostat needs checked. I have since, ran the motor for over 20 hours, with over 15 different starts, up and down the ICW in Florida, travelled over 70 miles, and have not had the least bit of motor problem. But, I was glad to have ordered the extra starter (since I switched it out, in 20 minutes and was going again), without needing to call for a tow.
Is it much work to pull the head? I have never done it. And I guess the pattern of re-tightening the headbolts and the torque is in the manual? Right? And, what does scarring look like on the cylinders? Are there any pics on youtube that show a scarred cylinder. Wouldn't I have to pull off the block in order to replace the rings? Does a compression test simply tell me if the rings are shot? It still starts easily now on the first turn of the key.
Is that a good indicator of good compression? The rating of the 3GMD is 20 HP at 3400. The RPM goes to 3600, when running at full throttle. And I move at 7.3 knots. Haul speed is 7. Thanks so much!

If it's running ok now maybe you dodged a bullet. Keep a close watch on oil consumption. Diesels will always need a top-off every 20 hours or so but any more than that and you should suspect some significant cylinder scoring. The next time you're not out cruising, maybe pull the head and check it out. Scoring looks like thin lines in the walls. The engine will still run with some scoring but you should see a lot of smoke if it's in need of immediate repair. As mentioned by dem above, you might want to re-ring and hone the cylinder walls. A danger is that the pistons and rings may have been damaged which can result in catastrophic failure and trashing the engine entirely. Also, the bearings may have overheated and become out of round slightly but this is probably not true unless you really baked it.

Gasket sets and rings for these are not terribly expensive. I recently re-ringed my 3GM30F for around $400, including a complete gasket set with a lot of extra gaskets that may be useful in the future. Before I bought this boat, the new engine had sat for 6 years. The rings never seated correctly. I was able to pull the pan right in the boat. If you can get the pan off with engine in place, it's not a difficult job. You can mount a mirror in the bilge so you can see what you're doing. Mark all the rod caps (they are probably numbered already), make sure you put them back in the same direction they came out so the oil weep holes are in the right direction. When in doubt, label it. Taking pictures before you take stuff apart with a dig. camera is also often helpful. The objective is to have no leftover parts. A ball-end hone is recommended but I like an old style stone hone. Both are fairly cheap. You will also need a ridge reamer if there are a lot of hours on the engine.

If you need a ridge reamer then you ought to check as to whether you need oversized rings. Also check all the journals to see if you need new bearings. I would only leave the old bearings if the engine has very few hours on it and the old plastigage as ok.

To do this kind of work you really need the shop manual for this engine as well. All that said, if you have never done this kind of work and don't feel confident about doing it, it would probably be better to yank the engine and bring it to a shop.

Based on my research, and forum responses that seem well reasoned, I need an oil with an hot viscosity of 40 or 50, and a cold viscosity as low as possible (for ease of starting and initial start flow). Something like 15w40 in conventional, or 5w40 in synthetic. Pretty much confirms what most people recommend.

Back to the original topic.... I've been using Shell Rotella 15w40 for 35 years in all my diesel and most of my gas engines (old school low rev type). Only in the very coldest part of winter do I use a 10w30 but then the temps are low enough that its rather hard to sail in unless its an ice boat.

Its my understanding from a local ag and desiel repair outfit that in recent years there have been changes to Rotella T and it may not be as good as it used to be... Something about the Chinese (imagine that!) now manufacturing, but I haven't been able to verify that as yet. He will not use it anymore and is a Dodge Cummin fanatic, but works on any diesel. Like me he swore by it for years. I haven't changed out from it yet and trust me I will if needed.

Without a thermostat sometimes (depending flow characteristics) the flow can be too much and prevent full operating temp...

I supposed it could be possible that the stay time around the cylinder could be too short... too short to absorb enough heat.... but I really don't know... I think it might rare though or maybe impossible for a design to be that way and actually get real hot.

Is yours raw water cooled or freshwater cooler (i.e using a heat exchanger)? It would help in determining the culpret if we knew. (forgive me if you already stated that somewhere, but I haven't seen it if you did)

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